Well, yolk color is an excellent indicator of the hen's diet.
If they eat more natural pigments, it affects and changes the yolk's color.
My girls have free run of a fenced in acre and they give eggs w/ dark orange yolks.
They are great foragers and eat lots of grasshoppers, grubs, vegetation, earthworms, etc.
They also get fresh fruits and veggies from our garden.

Something must have changed in their diet if this is suddenly occurring.
Do you notice any changes in your birds' behavior?

Well, yolk color is an excellent indicator of the hen's diet.
If they eat more natural pigments, it affects and changes the yolk's color.
My girls have free run of a fenced in acre and they give eggs w/ dark orange yolks.
They are great foragers and eat lots of grasshoppers, grubs, vegetation, earthworms, etc.
They also get fresh fruits and veggies from our garden.

Something must have changed in their diet if this is suddenly occurring.
Do you notice any changes in your birds' behavior?

I have been really searching for answers for you and the only thing that's come up regarding white yolks (besides diet) is an" infestation of cappilaria worms and certain diseases".
Credit thread by member sumi: https://www.backyardchickens.com/a/common-egg-quality-problems
Good luck, hope you can figure this out.